Engine developers continually balance the tightening emissions legislation with consumer demands for maximum fuel efficiency. In modern diesel engines, after treatment systems are deployed to reduce the engine-out emissions to the legislated tailpipe-out levels. While the after treatment systems are effective in reducing emissions, the true cost of their usage is the increase in fuel consumption required.
Advanced combustion modes including diesel premixed charge compression ignition (PCCI), homogenous charge compression ignition (HCCI) and low temperature combustion (LTC) offer the potential of reduced emissions while maintaining high engine efficiency. The lack of a direct combustion trigger has previously limited the widespread adoption of diesel PCCI and other advanced combustion modes. In laboratory testing, these strategies have been shown to be enabled through the flexibility gained through the use of flexible valvetrains. The control of these strategies is assisted by control authority over some of the inputs governing the combustion event, including the in-cylinder oxygen fraction.
Advanced combustion modes, such as PCCI, operate near the combustion system stability limits. In PCCI, the combustion event begins without a direct combustion trigger in contrast to traditional spark-ignited gasoline engine and direct-injected diesel engines. The direct combustion trigger can be replaced in some embodiments by the usage of model-based controls to provide robust control of the combustion phasing. The nonlinear relationships between the control inputs and the combustion system response can limit the effectiveness of traditional, non-model-based controllers. Accurate knowledge of the system states and inputs is helpful in implementation of an effective nonlinear controller.